1. Field of the Invention
The present invention relates to a rotation angle sensor, a torque sensor incorporating the rotation angle sensor, and an electrically driven power steering apparatus using the torque sensor, and, more particularly, to a rotation angle sensor which can detect rotational angles with high precision, and apparatuses to which the rotation angle sensor is applied.
2. Description of the Related Art
FIG. 12 illustrates a conventional rotation angle sensor, wherein a rotary drum 2, having disk-shaped magnetic portions, is affixed to a rotary shaft 1. A magnetic code, consisting of a plurality of magnetic north-south (N-S) poles, is formed along the entire outer periphery of the rotary drum 2.
A magnetic detecting sensor 4 is provided so as to be spaced from the outer periphery of the magnetic drum 2 by a predetermined gap P that is disposed therebetween, whereby the conventional rotation angle sensor is formed.
Such a conventional rotation angle sensor is constructed such that when the magnetic drum 2 is rotated as a result of rotation of the rotary shaft 1, the magnetic sensor 4 detects analog changes, that is changes in the magnetic forces of the magnetic poles, in order to detect the angle of rotation of the magnetic drum 2.
A description will now be given of a conventional torque sensor, wherein two such rotation angle sensors described above are mounted to a rotary shaft 1 having a drive shaft portion la and a load shaft portion 1b illustrated in FIG. 13.
The drive shaft portion 1a and the load shaft portion 1b of the rotary shaft are connected together by a resilient member (not shown), being a torsion bar.
The two rotary drums 2 and 3 are affixed to the drive shaft portion 1a and the load shaft portion 1b, respectively. They are connected towards an end of the drive shaft portion 1a and an end of the load shaft portion 1b that is connected to the drive shaft portion 1a, and are separated by a distance L.
A pair of magnetic sensors 4 and 5 are provided such that the magnetic sensor 4 is separated from the outer periphery of the rotary drum 2 by a predetermined gap P1 and the magnetic sensor 5 is separated from the outer periphery of the rotary drum 3 by a predetermined gap P2.
In such an operating shaft 1, by applying a torque to the drive shaft portion 1a that is greater than the torque applied to the load shaft portion 1b, the drive shaft portion 1a and the load shaft portion 1b can be rotated.
When the rotary shaft 1 is rotated, the load shaft portion 1b starts to rotate slightly later than the drive shaft portion 1a, due to the resilient member.
A slightly delayed rotation of the load shaft portion 1b results in a difference between the rotational angle of the drive shaft portion 1a and that of the load shaft portion 1b. The difference in the rotational angles is proportional to the rotational torque on the drive shaft portion 1a, so that when the difference in the rotational angles is large, the rotational torque on the drive shaft portion 1a is large, whereas when the difference in the rotational angles is small, the rotational torque on the drive shaft portion 1a is small.
Such a conventional torque sensor can detect the rotational torque on the drive shaft portion 1a by computing the difference between the rotational angles of the drive shaft portion 1a and the load shaft portion 1b through an integrated circuit (IC), which is not shown.
However, in such a conventional rotation angle sensor and torque sensor, rotational drums, being magnetic media, are directly mounted to the rotary shaft 1, so that when a large load torque is applied to the load shaft portion 1b, the drive shaft portion 1a and the load shaft portion 1b may become decentered. When decentering occurs, the amount of gap P1 between the rotary drum 2 and the magnetic detecting sensor 4 and the amount of gap P2 between the rotary drum 3 and the magnetic detecting sensor 5 change, so that the magnetic detecting sensors 4 and 5 cannot detect the strength of the magnetic field between the two rotary drums 2 and 3 with precision. This makes it difficult to make precise detections of the difference between the rotational angles.
Therefore, when such a conventional torque sensor is used in an electrically driven power steering apparatus of, for example, an automobile, the shafts 1a and 1b must be formed precisely and made highly durable, in order for the power steering apparatus to provide highly reliable power steering properties.
Accordingly, it is an object of the present invention to make it possible to overcome the above-described problems in order to provide a high-precision angle sensor.
To this end, according to a first aspect of the present invention, there is provided a rotation angle sensor comprising a rotary member having a gear portion at the outer peripheral portion thereof, the rotary member having a shaft-inserting hole at the center of rotation thereof; a code plate which engages the gear portion of the rotary member, the code plate having an information recording portion which rotates in response to the rotation of the rotary member; and a detecting element for detecting information written on the information recording portion; wherein when the code plate rotates as a result of rotation of the rotary member, the detecting element detects the information on the code plate in order to detect the rotation angle of the rotary member.
Although not exclusive, in a preferred form of the invention, the rotary member may comprise a first rotary member portion and a second rotary member portion, the first rotary member portion and the second rotary member portion being separately rotatable and having the same center of rotation; the code plate may comprise a first code plate portion and a second code plate portion, each having a gear portion which engages the rotary member and being separately rotatable; and the detecting element may comprise a first detecting element portion and a second element portion for detecting information on the first code plate portion and the second code plate portion, respectively. In this structure, when the first rotary member portion and the second rotary member portion are rotated by engaging the gear portion of the first code plate portion with a gear portion of the first rotary member portion, and by engaging the gear portion of the second code plate portion with a gear portion of the second rotary member portion, the information on the first code plate portion is detected by the first detecting element portion, and the information on the second code plate portion is detected by the second detecting element portion, whereby the rotation angle of the first rotary member portion and the rotation angle of the second rotary member portion are separately detected.
Although not exclusive, in a preferred form of the invention, the first code plate portion and the second code plate portion may have the same center of rotation, with one side of the gear portion of the first code plate portion and one side of the gear portion of the second code plate portion being disposed such that they face each other, and the other side of the gear portion of the first code plate portion having formed thereat the information portion associated thereto and the other side of the gear portion of the second code plate portion having formed thereat the information recording portion associated thereto, the outside diameter of each information recording portion being larger than the outside diameter of the gear portion associated thereto. In addition, the first rotary member portion and the second rotary member portion may be rotatably interposed between the information recording portions.
Although not exclusive, in a preferred form of the invention, the rotary member, the code plate, and the detecting element may be accommodated in a box-shaped housing, and the code plate may be disposed between the detecting element and the rotary member.
Although not exclusive, in a preferred form of the invention, the information recording portion of the code plate may be composed of a magnetic material with a plurality of magnetic poles, and the detecting element may comprise a magnetic sensor which reacts with the magnetic field of the magnetic material.
According to a second aspect of the present invention, there is provided a torque sensor comprising a rotation angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotation angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first rotary member and the second rotary member rotate to rotate the first code plate and the second code plate, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The torque sensor further comprises a first operating shaft and a second operating shaft, an end of the first operating shaft and an end of the second operating shaft being abutted against each other and connected by a resilient member, being a torsion bar. In the torque sensor, the first rotary member is supported by the end of the first operating shaft, and the second rotary member is supported by the end of the second operating shaft, in order to detect the rotation angle of the first operating shaft by the first detecting element and the rotation angle of the second operating shaft by the second detecting element, whereby the rotational torque on the first operating shaft is detected from the difference between the rotation angle of the first operating shaft and the rotation angle of the second operating shaft.
Although not exclusive, in a preferred form of the invention, a spring member may be provided at the inner peripheral surface of the edge of the shaft-inserting hole of the first rotary member and at the inner peripheral surface of the edge of the shaft-inserting hole of the second rotary member, the spring members resiliently pressing against the first and the second operating shafts in order to support the first rotary member by the first operating shaft and the second rotary member by the second operating shaft.
According to a third aspect of the present invention, there is provided an electrically driven power steering apparatus comprising a rotary angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotary angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first code plate and the second code plate are rotated as a result of rotation of the first rotary member and the second rotary member, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The electrically driven power steering apparatus also comprises a vehicle handle side steering shaft for supporting the first rotary shaft, and a vehicle wheel side steering shaft for supporting the second rotary member, an end of the vehicle handle side steering shaft and an end of the vehicle wheel side steering shaft being abutted against each other and connected by a resilient member, being a torsion bar. The electrically driven power steering apparatus further comprises a motor used for providing assistance in turning a handle. In the apparatus, the rotation angle of the handle side steering shaft is detected by the first detecting element, and the rotation angle of the wheel side steering shaft is detected by the second detecting element, in order to detect the rotational torque on the first operating shaft from the difference between the rotation angle of the handle side steering shaft and the rotation angle of the wheel side steering shaft, whereby when the rotational torque exceeds a predetermined value, the motor starts to operate to provide assistance in turning the handle.